This invention relates to a filter unit and a frequency conversion filter for use in selecting a particular signal in a situation where a plurality of signals different in bandwidth coexist and a common frequency band is shared.
A filter unit of the type is used, in a situation where a plurality of signals different in bandwidth coexist as input signals and a common frequency band is shared, to select a target signal after conversion of frequencies of the input signals into specific intermediate frequencies, respectively.
Referring to FIG. 1, description will be made as regards a conventional filter unit. In the conventional filter unit, input signals 111 and 112 are converted to have a same intermediate frequency and supplied to bandpass filters 71 and 72 different in filtering bandwidth from each other to produce output signals 121 and 122.
In the conventional filter unit of FIG. 1, however, the bandpass filters 71 and 72 are different in filtering bandwidth from each other. Therefore, either one of them alone can not be used in common to all input signals. In addition, the bandpass filters 71 and 72 has a same center frequency. Accordingly, if their inputs as well as their outputs are simply connected, it is not possible to assure attenuation characteristics of the bandpass filters narrow in filtering bandwidth. Therefore, the input and the output signals of the plurality of filters different in bandwidth from each other must be independent from each other. This requires blocks to be provided outside of the filter in order to separate the input signals as well as the output signals. Thus, miniaturization is difficult to achieve.
Referring to FIG. 2, the description will be made as regards another conventional filter unit. In the conventional filter unit, the input signals 111 and 112 are supplied in common to a selection switch 41 while the output signals 121 and 122 of the filter are connected in common to a selection switch 42. The selection switches 41 and 42 are switched by a selection control signal 201 on demand to select one of the bandpass filters which has a filtering bandwidth corresponding to a target signal. A reference numeral 101 represents a signal supplied to the filter while a reference numeral 106 represents a signal produced from the filter. The above-described conventional example is also led from a band split filter disclosed in JP-A 60-223304.
In the conventional filter unit of FIG. 2, the selection switches 41 and 42 are formed by active elements. In particular, if a relatively high frequency of several MHz or more is selected as the intermediate frequency, the bandpass filters 71 and 72 are generally formed by passive elements. Therefore, if all those components are accommodated in a same package for the purpose of miniaturization, sufficient miniaturization is difficult. Besides, there arises another problem of the increase in cost resulting from various factors such as the increase in the number of steps.